Why would you want to impose a 20kHz roll-off with a grid stopper resistor? The grid stopper is there to prevent parasitic HF/VHF oscillations and for no other reason.
I didn't look at your math but the numbers seem ok, use a 1 - 10K grid stopper and if you need to introduce a roll off for feedback stability you can do it in the driver stage.

Bandwidth in excess of 20kHz is a good thing if it doesn't come at the expense of something else like stability.

Most of my amps are designed to be flat to something considerably greater than 20kHz if possible.

__________________"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

12.1 k Ohms would put the response down 3dB at 200 kHz, which is usually plenty low enough. You're not trying to set the upper frequency open loop response with the stopper resistor- only trying to prevent oscillation at (usually) several MHz by reducing the gain above the intended passband.

High value grid stoppers are often used in guitar amps, partly to cope with poor layout involving long leads, and partly to limit the HF to less than the full audio range. In my view this is poor engineering. It makes them very sensitive to details of valve capacitance, and so encourages tube rolling as a crude way to adjust capacitance.

In hi-fi amps smaller grid stoppers are used simply to avoid VHF oscillation. Tone controls can be added separately if required.